Sensor arrangements, for example microphones, are used to record ambient noises or ambient sound. In order to provide a good quality of the recorded sound or in order to meet requirements of customers, a high degree of linearity, high signal-to-noise ratios (SNR) or the correspondence with a predefined spectral mask may be required for the response function of a microphone.
As is illustrated in the block diagram in FIG. 5, a conventional microphone 500 has a microphone diaphragm 502, for example a micro-electromechanical diaphragm (MEMS). The diaphragm 502 is deflected from a rest position by means of sound-induced pressure fluctuations and in the process generates an analog electrical signal which is amplified by means of an amplifier 504 or a read-out circuit, for example a source follower. The signal from the amplifier 504 is detected by a sensor circuit 506 at a sampling frequency Fs (510). The sensor circuit 506 has an analog/digital converter 512 which converts the signal from the amplifier 504 into digital signals. The sensor circuit also has a digital filter 514 which blocks high-frequency digital signals from the analog/digital converter 512 (low-pass filter). The filtered digital signals are converted into a customer-specific one-bit output signal 508 by a modulator 516 which is connected to the digital filter 514.
The low-pass filter is needed to stabilize the arrangement and to attenuate an excessive increase in the MEMS frequency response. A group delay is associated with this arrangement.
Owing to process variation and package variation, real microphones have a considerable variation in the frequency response. FIG. 1 shows a graph 100 in which the amplitude response 104 in units of dB is shown on the basis of the frequency 102 of the signals for different microphones 106, 108, 110 of the same design. It is clear from FIG. 1 that the lower frequency range, which is determined by the acoustic high-pass filter, is affected, in particular.
In some applications, ambient noises are recorded and evaluated at the same time using a plurality of microphones. For this purpose, the plurality of microphones are each arranged in a microphone array (MIC array) at a distance in a specific arrangement with respect to one another. As a result, a sound wave arrives at the individual microphones with a temporal offset. In order to make it possible to precisely evaluate the signals recorded by the microphones, the individual microphones should not have any fluctuations or should have only slight fluctuations in the frequency response with respect to one another. The frequency response implies no or only slight fluctuations in the amplitude response, phase response and the group delay of the recorded signals. The lower frequency range, for example up to approximately 4 kHz, is very important for such applications.
An attempt is currently being made, by means of circuitry measures, to keep the variation in the frequency responses of the individual microphones as low as possible. However, limits are imposed on this approach or signify a corresponding additional outlay.